feat: Tradier broker mirror — config-switch redundancy for Alpaca execution

tradier_broker/ mirrors every trading call the live loop makes on
AlpacaPaperClient (account, clock, positions, orders, submit incl.
bracket->OTOCO, cancel, replace, close) with identical signatures and
Alpaca-shaped returns, so trading_execution / exit_manager / flatten /
execution / sync run unchanged. Market data always delegates to Alpaca.

- BROKER=tradier (+ TRADIER_* env) swaps execution client in run_live_paper;
  default path untouched
- client_order_id -> tag preserves the lost-submit reconcile
- TradierApiError subclasses AlpacaApiError so the executor's
  rejection-vs-transport fork works as-is
- sandbox-verified: balances/clock/positions/orders, OTOCO preview
  (result:true), place -> tag lookup -> cancel round trip
- 18 unit tests (translation + client paths); executor integration checked
- docs/TRADIER-FALLBACK.md switch runbook

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/TRADIER-FALLBACK.md

@@ -0,0 +1,53 @@
+# Tradier broker fallback
+
+Redundancy path if the Alpaca live account stays blocked. `tradier_broker/`
+mirrors every trading call the live loop makes on `AlpacaPaperClient` — same
+method names, signatures, and Alpaca-shaped return dicts — so
+`trading_execution`, `runtime/exit_manager`, `runtime/flatten`,
+`alpaca_paper.execution` and `alpaca_paper.sync` run unchanged.
+
+## Switch (config only)
+
+```
+BROKER=tradier
+TRADIER_ACCESS_TOKEN=...       # sandbox or live token
+TRADIER_ACCOUNT_ID=...         # e.g. VA71549758
+TRADIER_ENV=sandbox            # 'live' for real money — explicit opt-in
+```
+
+`BROKER` unset/`alpaca` = today's behavior, byte-for-byte. Alpaca keys stay
+required either way: **market data (bars, screeners, quotes, news) always
+stays on Alpaca** — the Tradier client only replaces order execution and
+account/position state, delegating all data calls to the Alpaca client.
+
+## Verify before trusting
+
+```
+python3 -m tradier_broker.smoke                   # balances/clock/positions/orders
+python3 -m tradier_broker.smoke --preview AAPL    # broker-validates bracket form, never submits
+python3 -m tradier_broker.smoke --place-cancel AAPL  # full submit -> tag lookup -> cancel
+```
+
+Smoke refuses order exercises against the live base URL.
+
+## Mapping notes
+
+- Alpaca bracket -> Tradier OTOCO (entry + OCO take-profit limit / stop);
+  oto -> OTO; standalone gtc stop sells map directly.
+- `client_order_id` -> Tradier `tag` (lost-submit reconcile works via tag lookup).
+- Tradier positions carry no mark; `get_positions` prices them off
+  `/markets/quotes` (sandbox quotes are 15-min delayed — fine for redundancy
+  checks, and live tokens get real-time).
+- `replace_order` maps to PUT (moves stop/limit without cancelling — OCO stays
+  intact); Tradier cannot change qty on a working order (raises, cancel+resubmit).
+- `get_orders` is session-scoped at Tradier (today's orders) and status/symbol
+  filters are client-side; the live loop only queries today + held symbols, so
+  behavior matches.
+
+## Before flipping live money to Tradier
+
+1. Rotate the sandbox token (it transited chat) and mint a live token.
+2. Run a full paper week on sandbox with `BROKER=tradier` in the paper stack.
+3. Re-run the naked-stop drill from docs/LIVE-CUTOVER.md — leg-stripping
+   semantics differ per broker; verify a filled OTOCO's stop leg survives an
+   entry-order cancel attempt.